As early as 1960, Thomas Francis Jr. first noted that antibody responses to early childhood influenza virus infections are preferentially recalled later in life upon exposure to antigenically distinct viral strains. He coined the phrase ?original antigenic sin? to describe this phenomena. We recently demonstrated that human antibody responses to H1N1 and H3N2 are typically focused on epitopes that are conserved between contemporary viral strains and viral strains that circulated during an individual?s childhood. Importantly, we found that ferrets sequentially infected with older and contemporary influenza virus strains possess antibodies that have the same specificity as humans that were exposed to the same viral strains. Thus, ferrets are good animal models for studying how prior influenza exposures affect the specificity of antibody responses elicited against antigenically novel influenza virus strains. We have not yet explored how prior exposures influence antibody responses against seasonal influenza vaccines that include antigens from H1N1, H3N2, and influenza B viruses. This is an important consideration since the effectiveness of each influenza vaccine component differs among different aged individuals with distinct immune histories. We hypothesize that seasonal influenza vaccines elicit antibody responses that are biased towards the first viral subtype that an individual encounters early in childhood, and that preferential boosting of antibody responses against these antigens occurs at the expense of generating robust de novo responses. In Aim 1, we will address this question by defining the fine- specificities of serum antibodies isolated from vaccinated ferrets and humans with different viral exposure histories. In Aim 2, we will develop and apply new techniques to characterize B cell responses in ferrets before and after seasonal influenza vaccination. Finally, in Aim 3 we will determine if a novel mRNA-based vaccine establishes broader immunological memory compared to initial influenza virus encounters that occur via single viral infections and inactivated vaccines. Together, these studies will improve our understanding of how prior influenza virus exposures influence the generation of antibody responses against seasonal influenza vaccines and will test a new influenza vaccine that has the potential to elicit broad unbiased immune responses.